More details of the code
A error protection code used is known as an RS(59,63) code. This code is based on the use of 6-bit symbols, with 4 parity symbols used to protect 59 data-symbols in a 63-symbol block. The code is able to correct up to two errored symbols in any block.
To add more protection against burst errors, 6-way interleaving was also used to spread the 63 symbols in any block over the 378 symbols of each frame (this is the origin of the 378-word frame length). This is analogous to using 6 error correctors in parallel, each one working on every 6th word of the frame.
At an input bit error rate (BER) of 5x10–5 (which is 5 random errors in every 100,000 bits), the error-corrector reduces this to an output BER of 1x10 –9. The system still gives about an order of magnitude improvement with an input BER of 1x10 –3 and only fails to reduce the errors at an input BER of 1x10–2 - at which point the whole system is close to total failure because of the inability to achieve frame synchronisation in the decoder.
A characteristic of these types of codes is that their performance in respect of burst errors is superior to the worst-case random errors considered above. Error-free transmission is possible even with a burst of 67 bits in every frame – a mean error rate of 3x10–2.